GB2280802A - An improved detector circuit - Google Patents

Abstract

A detector circuit for a sensor, e.g. a proximity detector in an intruder alarm, comprises an operational amplifier (11) having an active filtering system 29 in its output. The filtering system 29 comprises a capacitor 30 connected between the amplifier output and a strobe input, and a capacitor 31 connected between the strobe input and a compensation input. The capacitor 31 serves as a damping capacitor, whilst the capacitor 30 acts to eliminate disturbances which would be caused by the sensor picking up the mains frequency. Voltage-dependent resistor 21 protects the op-amp input. <IMAGE>

Description

AN IMPROVED DETECTOR CIRCUIT The present invention relates generally to an improved detector circuit, and particularly to a detector circuit suitable for use in an intruder alarm or a switch.

Intruder alarms, especially those which provide an alarm indication of the proximity of an intruder, require careful calibration in order to ensure that they are adequately stable in their operation and do not provide a spurious alarm indication from a detection of, for example, the movement of animals, or resulting from temperature changes within the protected environment.

Known proximity alarms generally operate by radiating an electro-magnetic field into the protected environment, such field being in the optical, infra-red or microwave regions, It is also known to generate an ultrasonic field which is radiated into the protected environment.

All of these known systems suffer from the disadvantage of excess sensitivity and a potential lack of stability due to the possibility that they may be triggered by events other than those for which they are intended to operate.

The present invention seeks to provide a detector circuit suitable for use in an alarm system, or in any other system in which a sensor is provided to generate a varying voltage signal, and in which spurious triggering from the generation of alternating electro-magnetic currents or varying electro-magnetic fields can be avoided without any loss in detection reliability.

One known sensor is described in our earlier International Application GB 92/00362, which describes a detector responsive to the proximity of and/or contact by a detectable object, comprising a sensor body of insulating material electrically connected to an electrically conductive element in turn connected to a detector circuit responsive to the static potential difference across the conductive element and operable, if the said potential difference crosses a triggering threshold, to trigger generation of an output signal.

The present invention provides an improved detector circuit for use in such a system, but is not limited to such application and may, in fact, be utilised for the detection of variation in any DC voltage signal supplied from any sensor or transducer utilised in any control system or circumstances other than those particularly described hereinbelow.

Without prejudice to the generality of the present invention, therefore, it will be described hereinafter with reference particularly to its application in an alarm or switch detector circuit.

According to one aspect of the present invention, a detector circuit for a sensor of the type operable to produce a varying voltage signal comprises an operational amplifier to one input of which the varying voltage signal is applied, in which the sensitivity of the circuit to alternating current variations in the varying voltage signal is reduced by filtering means at the amplifier output.

This unusual arrangement of filtering means has particular advantages in the present invention by improving the stability of the output and the reliability of the amplifier response to a varying signal.

In a preferred embodiment the output filtering means include a capacitive coupling between the amplifier output and a strobe input of the amplifier. It is known that the output signal from an operational amplifier is 1800 out of phase with its input signal and by forming this capacitive coupling the response of the operational amplifier to an alternating current signal, typically of 50 Hz, is eliminated.

The sensitivity of the amplifier to higher frequency signals can be reduced by the provision of a capacitive coupling, forming part of the said output filtering means, between the strobe input and a compensation input of the operational amplifier. This capacitive coupling serves to damp the output signal so that it does not produce a triggering response to high frequency signals.

Preferably the input to the operational amplifier is connected to ground by a voltage-dependent resistor, the impedance value of which falls with an increase in potential difference across it whereby to limit the voltage variation at the input to the operational amplifier. This makes it possible to employ a sensor the voltage output of which may vary greatly, or which may have a relatively small variation, and nevertheless provide satisfactory sensitivity by protecting the operational amplifier against the large voltage variations by means of the voltage dependent resistor whislt nevertheless retaining the ability to detect the small voltage variations.

In another aspect the present invention provides a detector circuit for a sensor of the type operable to produce a response signal in the form of a voltage variation, and operative to trigger from a first, quiescent state to a second, active state on detection of a voltage variation greater than a predetermined threshold value, in which the sensitivity of the circuit to AC components in the input signal is reduced by filtering means at the output from the detector, operable to connect the output signal in phase opposition to an input or an intermediate stage of the detector circuit, thereby effectively cancelling signals above a certain minimum frequency.

The detector is thus sensitive to a DC variation in the voltage level from the sensor, and can operate to trigger an alarm or other output reliably in response thereto, without being triggered by unwanted AC signals which may appear on the line for example by induced coupling from the 50 Hz main signals generated by nearby electrical or electronic equipment.

One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a detector formed according to the principles of the present invention.

As can be seen in the drawing, the circuit is based on an integrated circuit operational amplifier generally indicated 11 having inverting and non-inverting inputs 12, 13 respectively. The non-inverting input 13 is biased by a voltage divider comprising resistors 14, 15 connected between a regulated voltage supply line 16 and a ground line 17.

The inverting input 12 is connected to a sensor terminal 18 via a resistor 19 and a protective diode 20 which protects the inverting input from any negative potential with respect to ground. Between the inverting input 12 and ground there is also connected a metal oxide varistor 21, which is a voltage dependent resistor acting, under quiescent conditions, to present a very high impedance and therefore very little loading to the input 12. The anticipated signal from the sensor terminal 18 is a voltage variation increasing from a low value in quiescent conditions to a higher value when a sensing event takes place.

Typically the sensor connected to the terminal 18 may be a sensor such as that described in our earlier International patent application referred to hereinabove, in which a dielectric panel or other member is intimately connected with a conductor such that a charge variation on the dielectric element, for example, by contact with a grounding object or by a change in the charge state due to proximity of a foreign body results in a change in the static field and thus a transient current flow detectable as a voltage variation for a limited time period. It has been found, for example, that a sensor of the abovedescribed type may be incorporated into a plastics material window or door frame of the type provided with a metal reinforcing core simply by connecting the terminal 18 to the conductive core. In such an arrangement the whole window frame itself then becomes the active sensor element which gives rise to a voltage variation at the terminal 18 due to currents induced in the conductive core by variation in the charge state of the plastics frame when a person approaches close to the window frame or comes into contact therewith. The detector of the present invention is thus particular suitable for use in a burglar alarm incorporating a sensor of this type.

In operation, then, a static charge state built up on the plastics component of the sensor is caused to vary by proximity or contact with a detectable object such as a human being in the vicinity, which causes the voltage on the terminal 18 to rise briefly as a transient current flows through the conductor embedded in the dielectric material. This transient rise in voltage lifts the voltage at the inverting terminal 12 and gives rise to an output from the operational amplifier 11 when the voltage at the inverting input 12 exceeds that applied to the non-inverting input 13, the value of which is determined by the value of the resistor 14 and 15.

The output from the amplifier 11 is at a "high" value when in the quiescent state, and this signal is applied via a resistor 22 to the base of a NPN transistor 23 which is thus held in a normally conducting state. The transistor 23 is connected in series with the coil 24 of a relay the contacts 25 of which are normally closed.

When the amplifier 11 detects a rise at the input 12 the output from the amplifier goes "low" causing the transistor 23 to become non-conductive and this results in the normally-closed contacts of the relay 24/25 opening. This also provides a fail-safe arrangement since if the power supply to the detector circuit fails the relay is automatically de-energized and the contacts open to indicate an alarm condition.

A further protective diode 26 is provided between the transistor 23 and the supply line 16 to protect the transistor 23 from a back EMF which is generated as the magnetic field in the relay coil collapses when it is turned off. The collector of transistor 23 is also connected via a resistor 27 and a light-emitting diode 28 to the ground line 17 so that as the transistor 23 is turned off and its collector voltage rises, the lightemitting diode 28 is illuminated to indicate that a detection has taken place.

In order to prevent the amplifier 11 from producing a spurious output by detection of an alternating current signal which does not signify that an alarm event has taken place at the sensor, and which may for example be due to inductive coupling with nearby electrical or electronic equipment, the amplifier 11 is provided with an active filtering system generally indicated 29 in its output. The filtering system 29 comprises a capacitor 30 connected between the amplifier output and a strobe input, and a capacitor 31 connected between the strobe input and a compensation input to the amplifier. The capacitor 31 serves as a damping capacitor, whilst the capacitor 30 connected between the output of the operational amplifier and its strobe input (which is usually used for controlling the operational amplifier from an external source) acts to eliminate disturbances which would be caused by the sensor picking up the 50 Hz mains frequency. The filtering effect is achieved, as mentioned above by virtue of the fact that the signals at the strobe terminal and at the output are in phase opposition so that the capacitive coupling achieved by the capacitor 30 effectively causes alternating signals to be cancelled out. In this way the operational amplifier 11 acts reliably to provide an output signal only when experiencing a transient change in voltage at its inverting input 12 signifying a detection of an alarm event on the terminal 18.

Claims (6)

1. A detector circuit for a sensor of the type operable to produce a varying voltage signal, comprising an operational amplifier to one input of which the varying voltage signal is applied, in which the sensitivity of the circuit to AC variations in the varying voltage signal is reduced by filtering means at the amplifier output.

2. A detector circuit as claimed in Claim 1, in which the output filtering means include a capacitive coupling between the amplifier output and a strobe input of the amplifier.

3. A detector circuit as claimed in Claim 1 or Claim 2, in which the output filtering means include a capacitive coupling between the strobe input and a compensation input of the operational amplifier.

4. A detector circuit as claimed in any of Claims 1 to 3, in which the input to the operational amplifier is connected to ground by a voltage dependant resistor, the impedance value of which falls with an increase of potential difference across it whereby to limit the voltage variation at the input of the operational amplifier.

5. A detector circuit for a sensor of the type operable to produce a response signal in the form of a voltage variation, and operative to trigger from a first, quiescent, state to a second, active, state upon detection of a voltage variation greater than a predetermined threshold value, in which the sensitivity of the circuit to AC components in the input signal is reduced by filtering means at the output from the detector, operable to connect the output signal in phase opposition to an intermediate stage thereby effectively cancelling signals above a certain minimum frequency.

6. A detector circuit substantially as hereinbefore described with reference, and as shown in, the accompanying drawing.